1. Field of the Invention
The present invention relates to the recovery of condensed metal from metal processing furnace, and more particularly, to methods and apparatus for directing a stream of dry ice against a surface of a processing chamber to displaced metal condensed on the surface.
2. Description of Related Art
The Super Alloy industry manufactures numerous types of alloys utilized primarily in the aerospace industry. Many super alloys contain relatively high percentages of magnesium (Mg), as well as, chromium (Cr), cobalt (Co), and cadmium (Cd). Magnesium is a very light, silver-grey metal that tarnishes slightly in air. When finely divided into powders or dusts, magnesium readily ignites upon heating in air and burns with a dazzling white flame. Normally, magnesium is coated with a layer of oxide (for example, MgO) that protects it from reacting with air and/or water.
Magnesium-containing alloys are typically manufactured in Vacuum Induction Melting (VIM) furnaces, under vacuum, in the absence of oxygen and moisture. While these furnaces may have slightly different configurations, they generally have four basic components: a Main Chamber, where melting occurs; a Dome, that is, an area over the main chamber—often the top of the dome is 18-30 feet high and may retract to allow overhead cranes to move the crucible in and out of the furnace; a Crucible, that is, the pot that alloys are actually mixed and melted in; and a Stanchion, that is, the mechanism that locks the crucible in place and physically lifts, tips, and pours the liquid metal into, for example, molds. Further features of a VIM furnace are described in an online materials processing database that appears at http://www.azom.com/details.asp?ArticleID=1505, which is incorporated by reference herein.
As the metal, for example, a super alloy, such as, magnesium, and other components of the alloy are melted at temperatures that exceed 2800 degrees F., a vapor is produced that upon contact with the furnace dome and walls, hardens into a brittle material that the industry often refers to as “condensate.” The magnesium portion of this condensate is not coated with the layer of oxide that normally protects the magnesium from reacting with air or moisture. As a result, the condensate is typically very reactive and prone to creating fires. If the condensate is not regularly removed from the VIM furnace, the condensate may flake off and fall into the pots of melting alloy and cause the resulting batch of alloy to be “off spec” and unusable for its intended purpose. The safe removal of magnesium-containing condensate has plagued the industry for years.
Prior Art Methods of Condensate Removal
In the past, three primary methods of condensate removal have been employed: Burn Off, CO2 Blasting, and Manual cleaning.                Burn Off: Burn off is the process of releasing the vacuum in the VIM and introducing oxygen into a furnace at melting temperature. The introduction of oxygen results in the magnesium igniting and burning off. This method is typically minimally effective. Since the burn off is typically surficial in nature, a significant amount of the reactive magnesium is often unaffected, and still present in the furnace.        CO2 Blasting: The use of solid CO2, or dry ice, blasting has been used successfully for the removal of magnesium-containing condensate for a number of years. The dry ice is used as an abrasive blast medium that is delivered under high air pressure. The dry ice typically “explodes” upon impact with the condensate and the furnace substrate and sublimates. An added benefit to the use of dry ice as a blast medium is that the sublimated CO2 gas displaces the oxygen in the area and helps reduce the likelihood of fire. This is significant in that, during the dry ice blasting, the condensate may be broken into a fine dust that is potentially pyrophoric (that is, capable of igniting spontaneously in air).        While effective, the dry ice blasting method requires the individual performing the work to do so with supplied breathing air equipment to prevent the inhalation of high concentrations of metal dust and to ensure an adequate oxygen level to support life and heatlth. The supplied breathing air equipment also inhibits mobility in the event of an emergency. Moreover, the amount of dust generated during this operation greatly reduces visibility, and, in the event of either a fire or explosion, presents a real obstacle for an emergency evacuation of the furnace work area.        Manual Cleaning: Manual cleaning of VIM furnaces requires that individuals enter the furnace and use non-sparking tools to physically scrape the condensate from the walls and ceiling of the furnace. While this method is very effective, it is also the most dangerous. It still requires the use of supplied breathing air equipment and, even when using non-sparking tools, the friction generated during the scraping process routinely ignites the magnesium component of the condensate and generates fire. A number of individuals performing this manual operation have received severe burns while doing so.        
Due to the shortcomings and disadvantages of the prior art, there is a need in this art to provide a safe, effective system for removing condensate from VIM furnaces and related furnaces. In fact, failing to locate such a system in the industry, one leading supplier of super-alloy approached the present inventor and requested that the applicant develop and provide such a system. Aspects of the present invention provide the desired system, which has had a long-felt, but unmet need in the materials handling industry.